In the back of his police car, Primus had an aimless stare, didn’t respond to questions, and wouldn’t play with his ball.

The 3-year-old German shorthaired pointer had been working with the Sheriff’s Office in Broward County, Florida, for two years, sniffing out weed, cocaine, heroin, and other drugs. Now he was overdosing.

On October 27, Primus and two other police dogs, Packer and Finn, trained to find hidden cash, had spent the morning investigating an empty house as part of a Drug Enforcement Agency investigation into a heroin ring. The three dogs are now the poster pups for the dangers of fentanyl, a powerful synthetic opioid that can kill a human just from skin contact, and a dog just from a sniff.

“Everywhere that you could possibly try to hide drugs is where the dogs try to sniff,” Detective Andrew Weinman told BuzzFeed News. “Things are hidden in some pretty ingenious places.”

The dog’s handlers had first checked the house for anything that might hurt the animals, like broken glass, spilled chemicals, or other obvious dangers. Then the dogs, led by Primus, entered on leashes and began their search.

“They didn’t really find anything, aside from a little marijuana,” Weinman said. But just to be cautious, the Sheriff’s Office handlers, Dustin Thompson and Julie Fraley, always stay with the dogs for a half hour after a search to watch for signs of problems. After a few minutes, it was clear Primus was in trouble.

“He was just staring out into the distance without seeing his handler. He wouldn’t play with his toy, wouldn’t drink water, was kind of leaning on the back of the car,” Weinman said.

Soon it was clear that Packer and Finn were affected as well. It wasn’t clear what had happened. Dog overdoses from different drugs look very much alike, and the cops hadn’t found any drugs, anyway. The K-9 team drove to an animal hospital two minutes away.

The call came just as they arrived that the rest of the investigation team had discovered fentanyl in the ceiling of the house, in a box far above dog nose level. “It was nowhere near where these dogs could sniff,” Weinman said.

In September, the DEA issued a nationwide warning to police officers about fentanyl, a chemical cousin to morphine, heroin, and the opioid painkillers responsible for a nationwide overdose epidemic that took around 33,000 lives in 2015. Fentanyl is 30 to 50 times as potent as heroin and has contributed to a sharp uptick in US overdose deaths since 2013, when it first began to show up in illicit heroin. Queries from BuzzFeed News to state public health agencies suggest that fentanyl-related deaths now exceed those from heroin in more than a dozen states.

The DEA warning came after two Atlantic City police officers overdosed and nearly died after smelling a puff of an evidence bag containing a mixture of heroin, cocaine, and fentanyl.

“I felt like my body was shutting down, like I was going to stop living,” Atlantic County Detective Eric Price said in a DEA video about the incident.

Fentanyl is potent enough that just two milligrams — the size of a pinch of sand — can trigger an overdose in an adult.

What saved the Atlantic City police officers was the same thing that saved Primus: an injection of naloxone, a drug that blocks the brain receptors for opioid drugs.

“What we think happened was that some surface where there was mixing of the drugs, a couch or a coffee table, became contaminated with just a little fentanyl,” Weinman said. “The dog might have sniffed it, or licked it off its foot or even just had contact with the pads on their feet.” If the dogs’ toy balls had rolled on a contaminated floor, then they might have swallowed the fentanyl that way.

“It changes the way we have been doing business for a long time,” Weinman said. “Even a microscopic amount could be dangerous.”

Fentanyl busts are now a fact of life for narcotics investigators. The DEA reported the seizure of fentanyl in the arrest of a heroin ring near Hartford, Connecticut, on Thursday, for example. Investigators might have to wear masks, gloves, even full hazmat suits to conduct searches and bag up evidence in such cases.

For dog handlers, this is a real dilemma, Weinman said, since the dogs have to sniff the evidence to do their job. “If we have to do it again, we could inject the dogs with naloxone, we have been trained for that,” he said.

But fentanyl works so fast that some victims have been found with needles still in the arms after overdosing. Librarians in Philadelphia can identify the overdoses in their branch bathroom by the “thud” of a body’s collapse, according to a Philadelphia Inquirer report.

Primus, Packer, and Finn, who was just a trainee on the day of the overdose, were recovered and back home that afternoon. They're still on the job now.

Police teams across the country are asking whether their dogs should be trained to find fentanyl. For Weinman, it’s an agonizing decision.

"I think the question is, do you find it first with the dog to save other people from becoming contaminated with it, which would put the dog at risk. Or do you let it go undetected and let someone else stumble into it,” Weinman said. “Which one is better? I don’t know the answer.”